i don't see an automated way to do that. the preview property is unchangable at none.
there must be a better way but instead of spending hours (if you don't get a better answer in a few days), in the authoring environment play to or drag to the part of your video you want to use as a poster frame. stop your video (if it was playing), right click the video screen and click export frame. you'll save an image you can that you can import into flash and place in a layer over your video. convert the image to a movieclip so you can control it with actionscript.
Thanks for that. Your solution works and I was able to create the poster frame. I was following a tutorial of CS3 but something has changed in CS5 and it´s very easy the way you suggested. I´m now having other problems getting the dimensions of the flv which I need in order to size my video on my webpage properly.
you can retrieve the video size in a metadata listener.